Currently, there are several concepts known in the art which address the problem of increasing the resolution capabilities. According to a first example, off-axis illumination in the projection system of the projection apparatus together with sub-resolution sized assist features is used. In a second example, the concept of alternating phase shift masks is employed so as to enhance the resolution capabilities of the projection apparatus.
In order to achieve dimensional accuracy of the mask pattern during imaging on a substrate, sub-resolution sized assist features or structures for optical proximity correction can be included in the mask pattern. These features are additional structures as, e.g., serifs or hammerheads, or are placed in close proximity to the original mask features. Size, shape and placement of these structures are usually determined by using a simulation model of the photolithographic projection. Such a simulation model is usually called an OPC model or model for optical proximity correction.
Usually, a model for optical proximity correction requires input data which can be derived by a measurement using scanning electron microscopy (SEM). In this respect, a wafer can be processed and the resulting pattern after optical image projection on a resist layer can be measured with the scanning electron microscope. In order to provide quantitatively reliable input data, errors during SEM-measurements should be low.
Accordingly, there is a need in the art to increase and/or correct errors in scanning electron measurements during measuring structural dimensions of an integrated circuit for optical proximity correction.